optocoupler resistor

Thread Starter


Joined Mar 31, 2010
i am using hcpl2630 opto. my input is 3.3V and i need to put a resistor at cathode to gnd, to limit the current.how do i fix its value.how do i get this opto's CTR value?


Joined Sep 26, 2009
The datasheet for this device has all the info you need to figure things out....


B. Morse


Joined Jul 23, 2009
I disagree with the sergeant. If you look carefully on the first page of the datasheet, they are recommending that you use a minimum of 6.3 mA, so that over time as the LED output diminishes by up to 20%, you'll still have the same output reception. Notice that 6.3mA * (1-0.2) = 5mA. The max input voltage of the device is 1.4 V forward on the LED, so R= (3.3V-1.4V)/6.3mA = 301 ohms, which is a standard 1% value.

You can probably get away with using 380 ohms, but an old engineer's rule of thumb is always design for the worst case. If you use 300 Ohms, you're covered; if you use 380, you may or may not have problems now or later as the Opto degrades, which they all do over time (mainly the LED section).
Kamran Kazem